1. Field of the Invention
The present invention relates to a collimator lens which is used in an optical recording/reproducing apparatus such as an optical disc system or a photomagnetic disc system.
2. Description of the Prior Art
In an optical disc system, for example, a collimator lens is used so as to convert an incident light from a semiconductor laser as a light source into a parallel beam and to project the parallel beam on an objective lens.
Since collimator lens in such an apparatus must collimate the incident light from semiconductor laser at low aberration, the residual wave surface aberration in rms value of the lens is required to be about 0.07.lambda. (.lambda.: wavelength) or less. Unlike objective lens, since collimator lens is fixed to an optical system and not provided with a servo mechanism, heat-resistant property is required so that deterioration of the optical performance of the lens due to heat may not arise. Of course, collimator lens is also required to be of small size, light weight and low price.
Such a collimator lens as known in the prior art is constituted by two ordinary spherical lenses with refractive index being uniform. In this case, however, the four lens surfaces must be polished into spherical surfaces. Moreover, the polishing is very difficult if the lens diameter is small. Further, technique of high degree is required in combining a plurality of lenses, such as sticking or centering. As a result, the lens is of high cost.
A plastic lens with non-spheric surface being in the limelight in recent years is weak to heat in comparison to glass or the like so it does not have reliability in the configuration accuracy during its use. Also the cost becomes high to form the non-spheric surface.